While a motor control device is driving a motor, if an abnormality occurs in the motor or the motor control device, it is necessary to immediately stop the motor for safety reasons. As a stopping method used during such an immediate stop, there is a dynamic braking method using brake resistors, where resistors are connected between motor terminals so as to short-circuit the motor terminals, thereby stopping the motor.
During a dynamic braking stop using brake resistors when a motor crash stops, an excessive current flows, which causes damage to the motor or demagnetization of magnets in the motor. Therefore, in order that the current that flows into the motor does not become excessive, an appropriate dynamic brake resistor is selected. With the selected dynamic brake resistor, when the rotation speed of the motor is reduced corresponding to the characteristic (that is, the constant) of the motor to which the dynamic brake resistor is connected, deceleration torque becomes small, and the deceleration period of time from the starting to stopping of the deceleration tends to be long. Therefore, heretofore, reduction of the motor stoppage distance during an immediate stop has been difficult.
Patent Literature 1 describes a technique in which, in a motor control device, when an overcurrent of a motor is transmitted to brake resistors by turning on a brake relay when braking the motor and causing a short circuit between motor windings, the duty ratio of the switching operation of the switching element on the upper side of the inverter unit is controlled. Therefore, the amount of overcurrent consumed by the brake resistors changes in proportion to the rotation speed of the motor. If this technique according to Patent Literature 1 is used, when the rotation speed of the motor is high, because the amount of overcurrent consumed by the brake resistors is increased, it is thought that a breakdown of the motor and the brake relay due to overcurrent can be prevented even when the capacity of the brake resistors is reduced.
Patent Literature 2 describes a technique in which, in an inverter control device, when a servo motor is immediately stopped, the current that flows into the dynamic brake resistors is controlled such that it is kept constant. When using this technique according to Patent Literature 2, because the rotation speed of the servo motor decreases linearly, it is assumed that the coasting distance of the servo motor from when the immediate stop command is output to when the servo motor actually stops can be shortened.